Why wouldn't perf mode feel smoother?
It's 40 vs 60fps.
The purpose of a 40fps mode is to deliver higher visual fidelity that wouldn't be attainable at 60fps without the game feeling you're wading through treacle, which is how 30fps feels.
Test it against that. It's night and day, feels way smoother than a measly 10fps uplift would make you think due to the huge difference in frametime. It's much easier to get used to coming from 60+fps as well compared to how jarring 30fps feels.
I used 40fps unlocked with VRR in Spiderman, that way you get higher fidelity but the game still pushes above that whenever possible while VRR takes care of the stutter that would normally occur with fps fluctuations.